1. Field of the Invention
The present invention relates to implantable medical devices and more particularly to medical devices that are designed to be surgically or endoluminally placed in a body.
2. Description of Related Art
Medical devices designed to be introduced through catheter-based delivery systems or through trocars are often deployed using various remote visualization techniques, such as x-ray imaging, fluoroscopy, ultrasound, and/or video imaging.
It has been determined that devices made from certain microporous polymers, such as expanded polytetrafluoroethylene (PTFE), sometimes are difficult to properly visualize using certain remote visualization techniques because air trapped in the microporous polymer can distort remote images. Most porous materials will eventually wet-out with body fluids following implantation, although this process may take time. In the case of expanded PTFE, its hydrophobic nature can vastly slow the process of replacing air with fluid following implantation—which can lead to poor initial visualization following implantation.
Expanded PTFE is now a preferred material for use with many implantable surgical and interventional devices, such as vascular grafts, implantable sheet materials, stent-grafts, embolic filters, and various occluders including septal occluders. As use of this material has increased, it has become evident that these devices often do not provide optimal initial visual clarity under ultrasound, video imaging, and direct visualization.
Ultrasonic imaging is a somewhat vexing problem for implantable porous materials. “Sound” is generally defined as a periodic disturbance in fluid density, or inelastic strain of a solid, generated by a vibrating object. In the case of “ultrasound,” it is generally defined as sound with a frequency of over about 20,000 Hz. The velocity of ultrasound waves depends on the medium through which they propagate. The velocity of sound through air is about 330 m/sec; the velocity of sound through water is about 1480 m/sec; the velocity of sound through muscle is about 1580 m/sec. While liquids tend to transmit ultrasound waves, air tends to absorb such waves. As a result, the presence of air in an implantable membrane introduces a disruptive layer that will interfere with normal ultrasound wave transmission. While it is recognized that these problems can be corrected by replacing the air in the porous material with liquid, this process has generally been addressed through the slow wetting-out of the porous material over time following implantation.
For some applications, this process of slow wetting-out may be undesirable. With the growing advent of remotely delivered devices, more and more comprising a membrane attached to an expanding frame, there is a need for instantaneous exact visualization of the device prior to and immediately following implantation. Devices such as fluoroscopes and x-rays can provide such visualization, but the harmful radiation these devices deliver to patients and medical personnel make them less desirable for daily use. Due to its very low side-effect risks, ultrasound visualization would be a preferred method of visualization, but only if the remotely deployed devices can be instantly visualized without interference. To date, no entirely suitable method of instantly ultrasonically visualizing a device incorporating a porous membrane has yet been developed.
Visualization and wet-out issues are discussed in a number of existing patents. For instance, in Japanese Patent 10-244611 to Oga it is recognized that expanded PTFE implantable sheet material has a number of problems, including that: it cannot be seen through; it reflects light, causing glare problems for surgical staff; and it cannot be effectively probed with ultrasound. The patent teaches that these problems can be corrected by providing an expanded PTFE center layer that is pre-impregnated with an aqueous liquid and two outer layers sealing the liquid impregnated layer. Liquid polyvinyl alcohol (PVA) may be included in the liquid impregnated layer. While this approach may solve visualization problems, it presents a number of other problems, including vastly increased manufacturing, packaging, shipping, and handling problems while dealing with a pre-wetted material.
In PCT Patent Application WO 96/40305 to Hubbard, it is again recognized that expanded PTFE cannot be seen through, it reflects light, and it is not suitable for ultrasound imaging. Hubbard teaches that the expanded PTFE can be pre-impregnated with saline, polysaccharides, gums and gels, glycerol/gum xanthan, sera/lipids, or the like, and then shipped wet. Again, this concept requires increased expense and effort in dealing with the manufacturing, packaging, and handling of a “wet” product.
Separate from visualization issues, a number of other patents suggest incorporating wet or wettable materials within implantable devices for various reasons of improved device performance. For instance, U.S. Pat. No. 4,193,138 to Okita teaches use of an expanded PTFE vascular graft with a water-soluble polymer in its pores. The polymer in the pores forms a bonded film of water, preventing adsorption of plasma protein, which is claimed to improve patency. Multiple types of cross-linked PVA are disclosed as a “swollen gel” in the pores of the expanded PTFE.
Similarly, U.S. Pat. No. 5,041,225 to Norman teaches an expanded PTFE membrane coated with a combination of a hydrophilic polymer and a complexing agent. The polymer is rendered water insoluble by the complexing agent, which also provides good protein bonding. PVA is taught as the hydrophilic polymer and various inorganic compounds, such as boric acid, sodium borate, etc., are taught as the complexing agents.
In U.S. Pat. No. 5,049,275 to Gillberg-LaForce et al., it is taught that a micro-porous membrane, such as expanded PTFE, can be changed from hydrophobic to hydrophilic by incorporating a vinyl monomer, such as PVA, polymerized within the pores of the membrane. This patent teaches that the membrane should be rendered hydrophilic to be used as a separation membrane in rechargable batteries, or in blood oxygenators, in bioreactors or for use in blood dialysis, or to support a liquid membrane, wherein a liquid which is imbibed in the pores of the microporous membrane is the medium through which transport takes place.
In U.S. Pat. No. 4,525,374 to Vaillancourt it is taught that an expanded PTFE membrane can be coated to render it hydrophilic by treating it with triethanolamine dodecylbenzene sulfonate and then dried. The patent teaches that the membrane should be rendered hydrophilic to maintain the existing (inert characteristics) surface properties of hydrophobic membrane filters and yet render these filters hydrophilic such that they can be used for fluid filtration, particularly for pharmaceutical processes.
In U.S. Pat. No. 5,755,762 to Bush it is taught that electrical conductivity can be improved by treating an expanded PTFE jacketed pacing or defibrillation lead with a wet-out agent, such as DSS, TDMAC, surfactants, or hydrogels. Likewise in U.S. Pat. No. 5,090,422 to Dahl et al., it is taught that an expanded PTFE pacing lead jacket can be treated with a “wetting agent, or surface modified” to allow wet-out and improve initial electrical performance.
U.S. Pat. No. 5,897,955 to Drumheller et al. teaches that a PVA coating can be provided on an expanded PTFE surface to aid in attaching various biological entities. U.S. Pat. No. 5,902,745 to Butler et al. teaches that a PVA treatment can be provided in the wall of an expanded PTFE cell containment device to aid in seeing the cells inside.
In summary, numerous concepts have been previously proposed for rendering a porous membrane wet or wettable for a number of functional reasons. However, particularly with regard to endoscopically deployed devices that mount porous membranes on some form of support frame, none of these previous concepts has taught or suggested an ideal solution to aid in the instant visualization of an implanted device that is highly effective, simple to implement, and does not urden the manufacturing, packaging, shipping, or handling of the implantable device.